The reason Kakashi Hatake's Chidori and the Fourth Raikage's Lightning Release Chakra Mode made them so fast was that they used Lightning Release to stimulate and activate the body's cells, increasing their overall speed.
But this method of increasing speed could not solve one problem: neural response.
A normal human's reaction time was roughly 0.3 seconds, while the very best athletes in his previous life could bring theirs down to around 0.1 seconds.
The retina received a signal and passed it to the brain cells. After responding, the brain cells ordered the body's cells to react accordingly. That was the process of human reaction.
Some people trained for years, preparing their bodies to respond to various situations and thus skipping the thinking process. For example, when a cup fell beside you and you casually caught it, that was the body's natural response, without any thought involved.
So the reaction formula for experts was: receive a signal, pass it to the brain cells, then pass it to the body's cells.
Normal human nerve signals traveled at eighty meters per second. In theory, there was no room for any enormous improvement, but there were always exceptions.
One example was the Fourth Hokage, Minato Namikaze, whose neural response was unrivaled in the Shinobi World. He threw a kunai, and all anyone else saw was a flash of yellow hair before every enemy was instantly killed. That was why he was called the Yellow Flash.
Shinsei's neural response was not weak, but who would not want to be stronger? He had no Sharingan, so he could only think of developing ninjutsu to make up for it.
Natural lightning traveled at approximately three hundred thousand kilometers per second, a full 3.75 million times the speed of signals in the human body.
If Lightning Release could simulate and replace the transmission of signals in the human body, then, if successful, his reaction speed would be unimaginable.
Of course, natural lightning certainly could not replace the body's signals. After all, it would not obediently follow the nerves. But Lightning Release Chakra had given Shinsei an idea. Although Lightning Release Ninjutsu displayed properties similar to natural lightning, it was fundamentally different. It was not real lightning, after all, but a form of energy.
Shinsei could not help imagining himself slaughtering his way through enemies after developing this ninjutsu, and a smile appeared on his lips.
But reality soon jolted Shinsei awake.
Developing this kind of ninjutsu required an extremely thorough understanding of the human body.
First, he needed to map every nerve. That sheer volume of data was beyond Shinsei's current capabilities. He had never done any research related to the human body.
Second, he had to decode the body's signals. Only by understanding the code behind their transmission could he replace them. It was like someone seeing intelligence on the battlefield ahead and reporting it to their commander. Only by understanding the language they used could you take their place and report to the commander. Then you still had to relay the commander's orders in a way the subordinates could understand.
Third, he had to replace the transmission of information. As mentioned earlier, after decoding the information, he still had to relay it in place of the original signals, ensuring that the brain cells and body cells accepted it.
For now, this ninjutsu was only an idea. It was not something one person could accomplish. If he seriously pursued its development, he would also need human experimentation. In any case, Shinsei would not take chances with himself. At most, he would use clones.
But if he really succeeded in developing this ninjutsu, he would find no opponent in the Shinobi World. It would be like The Flash descending upon it.
Sparks crackled and flashed in the basement. For now, he should focus on thoroughly understanding how Lightning Release activated cells. Shinsei's Lightning Release activation was still far from its upper limit.
The next day, inside the Hokage Building.
"Lord Hokage!" Shinsei greeted the Third Hokage with a proper bow.
"You're here, Shinsei." The Hokage smiled and nodded. "I have a rough idea about developing that sensory ninjutsu you mentioned yesterday. Give me another two days. I'll find some time to experiment before giving you an answer."
"So soon?" Shinsei was a little surprised. He knew how difficult developing this sensory ninjutsu would be. "Thank you, Lord Hokage!"
Hiruzen Sarutobi waved a hand and said with a smile, "You young people really do have lively minds, coming up with approaches no one else has thought of. I merely have some experience. By the way, you can choose a ninjutsu from the Scroll of Seals. Would you like to choose today?"
Shinsei scratched his head, looking earnest and embarrassed as he said, "Sorry for the trouble, Lord Hokage. If you have time, I'll choose today."
The Third Hokage nodded and stood up. "Follow me."
Shinsei hurried after him. After passing through room after room, Hiruzen Sarutobi opened a barrier and handed Shinsei a small scroll. Naturally, this was not the Scroll of Seals, but its table of contents and brief descriptions of its ninjutsu.
Shinsei confidently scanned the ninjutsu listed on it. Sure enough, Impure World Reincarnation was nowhere to be found.
At this point, Shinsei no longer cared much about the ninjutsu in the Scroll of Seals. He had already obtained Flying Thunder God, and Impure World Reincarnation was absent. As for the remaining ninjutsu, they either fell short of his Water Release: Kirin in power or were useless to him even if he obtained them, such as the entire series of Wood Release ninjutsu.
The Flying Thunder God Technique had probably only remained in the Scroll of Seals because almost no one could successfully learn it.
"I've chosen, Lord Hokage!"
Hiruzen Sarutobi looked at the ninjutsu Shinsei pointed to and nodded, then entered the room to transcribe it.
Before long, Hiruzen Sarutobi returned with a sheet of white paper covered in writing.
Shinsei took it. Sure enough, it was the Bringer-of-Darkness Technique.
Shinsei had made excellent progress in both taijutsu and ninjutsu, but he did not know a single genjutsu. He could only dispel them a little. In principle, as one of the Shinobi World's three major systems, genjutsu had no shortage of experts, and Shinsei's immense mental energy made him a natural genjutsu prodigy.
But Shinsei had always lacked instruction in genjutsu. Besides, no matter how well one cultivated the genjutsu system, the Sharingan kekkei genkai still stood above it. And among those who reached the pinnacle of Kage Level, or the even higher Six Paths Level, he had never seen anyone taken out by genjutsu.
Choosing the Bringer-of-Darkness Technique this time was an excellent addition to Shinsei's repertoire.
Speaking of which, this ninjutsu was definitely developed by the Second Hokage Tobirama Senju with the Uchiha Clan as his hypothetical enemy. The Uchiha Clan were recognized masters of genjutsu, and especially when facing the Sharingan, one could not meet their eyes.
Good eyesight, huh? Just seal off their vision.
After saying goodbye to the Hokage, Shinsei eagerly headed home to study his new ninjutsu.
Suddenly, a shuriken shot toward Shinsei at high speed.
Shinsei shifted his feet, instantly dodging aside, then turned to look.